Present day technology permits a large number of discrete electrical components (transistors, diodes, etc.) to be integrated on a single semiconductor chip that may be packaged to create an integrated circuit adapted for attachment to a circuit board. Extremely sophisticated electronic systems can be fabricated on a single circuit board by placing various integrated circuits thereon. There is now an effort to increase the level of performance of such systems by fabricating them from individual semiconductor chips, each placed on a silicon substrate which serves to connect the chips. Each chip is attached by a set of solder bumps on its bottom surface to a matching set of solder bumps on the substrate. The inductance of the joints between the chips and the substrate is much lower, as compared to the inductance which exists between a conventional integrated circuit and a circuit board, thus allowing for higher operating frequencies.
One difficutly that has limited the extent to which electronic systems may be fabricated from semiconductor chips placed on a silicon substrate is the high degree of accuracy required to place the chips. The typical diameter of each solder bump on the major surface of the substrate is on the order of 100 microns. The center-to-center spacing between solder bumps is generally on the order of 200 microns. In order to prevent short circuits, no more than a 25-micron placement tolerance between each solder bump on the substrate and each corresponding solder bump on the chip is permitted. Conventional placement equipment cannot generally achieve a placement tolerance of less than 50 microns.
Thus, there is need for a technique for placing a semiconductor chip on a substrate with a high degree of accuracy.